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A simple circuit, a USB TV tuner, your 
computer, and some powerful software 
combine to make an amazing software defined 
communications receiver. 



By George R, Steber 

his unusual radio can be used to receive 
conventional amplitude modulation (AM) and 
frequency modulated (FM) stations, as well as 
more specialized modes such as narrow band FM 
(NFM), single-side band (SSG), continuous wave 
(CW), and other signals. Its performance — while good — is 
slightly below the best shortwave listening (SWl) receivers 
or ham radios. However, we will show you how to improve 
its performance substantially with tunable filters. 

As it stands, it does a creditable job in many situations 
and has features that many other radios do not — like 
digital signal processing, spectrum analysis, and a waterfall 
display. As an added bonus, you will be able to receive 
frequencies from 24-1766 MHz in case you tire of the 
classic shortwave frequencies. 

In today's jargon, this receiver would be called a 
software-defined radio (SDR) — a concept made popular 
by the military. Typically, in an SDR receiver, an analog-to- 
digital conversion (ADC) is made on the RF signal, and the 
rest of the functions of a classic analog receiver are 
performed on the digital signals in software. These 
functions include tuning, filtering, and demodulation. 

Using software to replace hardware allows more 
versatility, provides more functions, and reduces hardware 
complexity. 

For this design, we will use a simple RF converter and 
a small inexpensive commercial digital TV module to 

40 NUTS S VOLTS July 2015 



provide the up-front hardware. The rest of the functions 
will be done on the computer. 

So, if you want to explore the world of shortwave 
radio and learn more about this technology, read on and 
definitely consider building your own software-defined 
receiver. 

Software-Defined SW Receiver 

Figure lisa block diagram of our shortwave SDR 
receiver. Starting from the antenna, there is an RF 
converter/mixer block. The purpose of this block is to 
perform frequency up-conversion of the shortwave signals. 
This is necessary because the next block requires RF 
signals above 24 MHz. Therefore, a simple up-conversion 
of the signals is performed using a mixer. Also in this block 
are some analog filters to prevent strong AM and FM 
signals from overloading the mixer and causing 
intermodulation distortion. 

The next block is a digital TV tuner — an amazing 
device that does most of the work. It is connected to the 
RF up-converter with a short coaxial cable to its antenna 
terminal. In this device, analog signals from the up- 
converter are tuned, changed to digital signals, and 
conveyed via USB to the last block. 

The final block is your home PC which performs all 
the necessary digital signal processing. It should be 



it was the simplest of times — it was the most complex of times. Indeed, 
much like those words, this shortwave radio project is probably the simplest, 
most complex one you have ever seen. It is simple because the main 
hardware components are low cost, readily available, and easy to assemble. 
It is complex because you need to download and install a special USB driver 
and other software on your PC — and get it all working together. Once the 
components are assembled and the software is installed, however, you will 
have a very versatile shortwave radio that covers the 1-30 MHz range, and 
provides many enjoyable hours of shortwave listening and experimentation. 
Since the software is free and the components are cheap, you should be able 
to build this receiver for around $25. 



Mini DVB-T USB Tuner 

The digital terrestrial TV age is in full bloom for 
millions of viewers around the world — especially in 
Europe, Analog terrestrial TV has been replaced by the 
new Digital Video Broadcasting Terrestrial (DVB-T) 
standard in many places. Viewers use mobile USB TV 
tuners with their computers for reception of HDTV 
broadcasts. Mobile tuners have compact dimensions — 
about the size of a memory stick. They plug into and are 
powered by the USB 2.0 port of your computer. 

Figure 2 is a photo of a DVB-T tuner package 
purchased on eBay for about $1 1 (including shipping). As 
you can see, a remote control and an antenna are 
included — ideal for watching TV on your laptop. 

We will not be using the DVB-T tuner for TV viewing 
in this project. Besides, it would not work in the US since 
the DVB-T standard is not implemented here. Instead, it 
will be repurposed to act as a wide-band tunable SDR 
receiver. Hence, we will not need the other items in the 

TV package. FIGURE 2. Low cost DVB-T package. 

July 2015 NUTS! VOLTS 41 




ANTENNA 



moderately fast, have 
some USB ports, and a 
sound card. The input data 
to the computer is entirely 
in digital form, coming 
from the TV tuner via 
USB. After applying digital 
algorithms for filtering and 
demodulation in the PC, 
the processed digital data 
is presented to the sound 
card — all in the digital 
domain. Finally, the sound 

card is used to convert the digital data to audio to drive 
speakers or headphones. Before we continue with details 
of the design, let's take a closer look at the little tuner 
device that makes this project possible. 



CONVERTED SIGNAL DIGITAL SIGNAL 
25-54 MHZ USB p0RT 



1-30 MHZ 




DVB-T TUNER 




PC SIGNAL 




R820T 


A 


PROCESSING 


RF CONVERTER 




RTL2832U 




SOUND OUTPUT 



FIGURE 1. Block diagram of SDR SW receiver. 




Post comments on this article and find any associated files and/or downloads at 

www.nutsvolts i com/index.php?/magazine/article/iuIy2015 i= Steber. 





FIGURE 3. RTL tuner (with R820T and Realtek 
RTL2832U inside) and BNC adapter cable. 



Figure 3 is a photo of the DVB-T stick we are using 
and a short interface cable that was bought separately. 
Most DVB-T sticks have two main chips inside; a digitally 
controlled tuner and an ADC that samples the baseband 
signal and outputs the samples to a host computer 
through a USB port. 

A very specific DVB-T stick is used in this project — 
one that has a Rafael Micro R820T tuner 1C and a Realtek 
RTL2832U inside. These so-called RTL sticks are quite 
common and available from many sources. Beware, 



though, as some sticks have an E4000 tuner or some 
other kind of tuner IC inside. Those will not work in this 
project. Make sure your stick has an R820T inside. 

The R820T tuner is crucial because it has the lowest 
tuning range — tunable down to 24 MHz. Other tuners do 
not have that low-end range. As we will see later on, this 
capability makes our shortwave up-converter easy to build. 

What makes the RTL stick so valuable is its inherent 
ability to demodulate FM signals and transfer the 
amplitude and phase information as raw in-phase and 
quadrature phase (l/Q) samples to the computer via USB. 
Annti Palosaari — a Finnish engineering student and Linux 
developer — discovered this special radio mode. 

Amazingly, this mode enables the tuner to output a 
stream of eight-bit l/Q samples at rates up to two million 
samples per second. 

Once this discovery was grasped, enthusiasm grew for 
the development of a cheap SDR. The group from Open 
Source Mobile Communication (Osmocom) — particularly 
Steve Markgraf — developed a basic set of drivers and 
utilities to communicate with the RTL dongle, After that, 
other software developers began writing code to use 
these drivers and provide user interfaces. One of those 
programs, SDR# (pronounced SDR Sharp} is probably the 
most popular one. We ll discuss it in more detail later on. 

RF Converter and Assembly 
Details 




FIGURE 4. Detailed schematic — shortwave radio 

RF up-converter. 



The front end of our 
shortwave receiver is a 
frequency up-converter. 
An up-converter is a 
circuit that adds a 
constant frequency to 
the received frequency — 
the one received at the 
antenna. An RF mixer 
and local oscillator can 
be used to make a 
simple up-converter. If 
we use a local oscillator 
frequency of 24 MHz, 
then the output of the 
mixer will contain the 
received signal plus 24 
MHz. Both the local 
oscillator and the mixer 
functions can be handled 
by a general-purpose 
SA612 device. 

If you search the 
Internet, you will see 
some designs that use 



42 NUTS S VOLTS July 2015 



down-converters with 125 MHz local 
oscillators. These circuits are overly 
complicated and harder lo build. They are also 
more prone to spurious signals and more 
costly. Our little up-converter costs quite a bit 
less and performs better in most instances. 

The design of the frequency converter is 
straightforward. However, there is one slight 
wrinkle as two different front-end filters are 
presented: broadband and tunable. This gives 
diverse users the option to utilize the SDR for 
different applications. The broadband fitter is 
low cost and works well over the entire 1,5-30 
MHz range, but has more noise due to inter- 
modulation from strong in-band stations. This 
option works well for strong AM SW stations 
from around the world without the bother of 
peaking a filter. 

On the other hand, the tunable filter has a 
narrower band and requires manual signal 
peaking, but has less noise and can dig out the 
weak stations. This option is better for receiving 
amateur radio and other low power stations. 

Figure 4 is the circuit for the 24 MHz up- 
converter. A parts list for the project is shown 
in Figure 5. Most parts for the receiver are 
readily available from sources like Mouser, Digi- 
Key, and Jameco. The SAG 12 is the only chip in 
the circuit, and is used for mixing and local 
oscillator functions. The up-converter frequency 
is determined by crystal CR1 , It needs to be 24 
MHz or slightly higher. The exact frequency is 
not critical, as an offset frequency will be used in 
the final software to compensate. So, any frequency in the 
range of 24 MHz-24,6 MHz will work fine. It is important 
that the capacitors C7 and C8 be good quality — like 
Class I NPO ceramic or even silvered mica — to minimize 
temperature drift. C7 is probably not needed. Leave it out 
if the circuit oscillates without it. 

The circuit is powered from eight to 12 volts DC and 
is regulated via the 78L05. Use a linear voltage supply as 
opposed to a switching type to avoid noise. Batteries work 
well too, as the current draw is small. 

As mentioned earlier, there are two RF input filter 
choices. Choosing the upper jumpers on jPI and JP2 
connects the bandpass filter (BP). You don't actually need 
to build both filters — only build the one you want. The BP 
filter is designed to remove strong stations from the AM 
broadcast band and FM band that would otherwise 
overload the mixer. 

The other choice is the tuned filter. It requires hand 
wound high Q coils and a variable capacitor to peak the 
signal. (See the Parts List for details.) For this filter, only 
specific bands are covered. So, with this option, if you 
want to cover the whole HF range, you will need to 



Parts List 



Qty 


Label-Value 


Designations) 


Description 


3 


0.01 pF 


C1.C2, C3 


Mono capacitor 


2 


0.1 pF 


C4, C5 


Mono capacitor 


1 


10 mF 


C6 


Electrolytic 


1 


0-10 pF 


C7 


Class 1 NPO ceramic- see text 


1 


47 pF 


08 


Class 1 NPO ceramic 


1 


15 pF 


09 


Ceramic capacitor 


1 


27 pF 


CIO 


Ceramic capacitor 


1 


150 pF 


011 


Ceramic capacitor 


1 


680 pF 


012 


Ceramic capacitor 


1 


220 pF 


013 


Ceramic capacitor 


1 


IK 


R1 


1/4W 5% resistor 


1 


27 


R2 


1/4W 5% resistor 


1 


150 


R3 


1/4W 5% resistor 


1 


10K 


R4 


1/4W 5% resistor 


1 


1 pH 


LI 


Fastran RF inductor, Mouser 


1 


2.2 pH 


L2 


Fastran RF inductor, Mouser 


1 


0.47 pH 


L3 


Fastran RF inductor, Mouser 


1 


1.5 pH 


L4 


Fastran RF inductor, Mouser 


1 


1.5 pH 


L5 


1ST, T50-6 core 


1 


0.4 pH 


L6 


9T, T50-G core 


1 


4 pH 


L7 


28T r T68-6 core 


1 


1 pH 


L8 


13T, T5Q-6 core 


1 


16 pH 


L9 


56T, T68-6 core 


1 


2 pH 


L10 


2GT, T50-6 core 


1 


78L05 


78L05 


5V regulator, Jameco 


1 


J310 


FET1 


FET 


1 


LEO 


Green 


LED 


1 


15-400 pF 


CV 


Variable capacitor 


1 


J1 


DC 


Jack 3.5 mm 


1 


J2 


RF OUT 


BNC type 


1 


J3 


AMT 


Two terminal 


1 


U1 


SA612 


RF mixer/osc, Mouser 


1 


01 


1N4001 


Diode 


1 


CR1 


Crystal 


24 MHz - see text 



Figure 5. Parts List for SDR SW receiver. 

provide switching between coils. The improved 
performance may be worth it. 

The variable capacitor CV needs to have a wide range 
— typically 15 pF to 400 pF. These capacitors are getting 
more expensive and harder to find, but can still be found 
on the Internet for around $15. 

The output "up-converted" signal is provided to )2 
(BNC) through a tuned output stage designed to match 
the 75 ohm input of the RTL antenna. Figure 6 is a 
breadboard for the tuned filter receiver. In general, this 
open layout is not recommended for RF circuits. For best 
performance, the mixer circuit should be built on a PCB 
(printed circuit board) and housed in a small metal box. 
Although a PCB design is not available, it should be easy 
to make — even for beginners, 

A short coaxial cable is connected from |2 to the RTL 
stick's antenna terminal. The RTL usually has an MCX type 
connector as its antenna terminal. Because an MCX plug 
is very small and difficult to solder, it is suggested that you 
buy a ready-made adapter cable — an MCX plug with a 
short RG31G cable to BNC. Figure 3 shows such an 
adapter cable. To reduce computer noise, a USB 

July 2015 NUTS1 VOLTS 43 




FIGURE 6. Breadboard of tunable filter shortwave radio. 



extension cable may be used to move the RTL farther 
away from the computer. 

Software Installation 
and Settings 

First of all, do not use any of the software that came 
with your RTL device. It was designed for a different 
application. For our radio, the RTL stick requires a special 
USB driver and a graphical radio interface. You will need 
to install Zadig for bulk interface drivers and then SDR# 
for the radio interface. Once Zadig is run, you will have a 
new USB driver named "Bulk-In, Interface (Interface 0)/' 
That is what the RTL device uses. 

After you install SDR#, you will be able to select this 
driver to use with your radio. Also note that — depending 
on your PC operating system — you may need Microsoft's 
Net 3.5 for the installation to work. 

The software installation discussed earlier is too 
detailed to present here. The following websites can take 
you through the procedure. Perhaps the best site that 
covers the entire installation is www.rtl-sdr.com/rtl-sdr- 
quick-starf-guide. Other sites to check are 
http://rtlsdr.org/softwarewindows and 
http://inst.eecs.berkeley + edu/"'ee123/fa12/rtLsdr.htmL 
The installation may seem a bit daunting when you are 
just starting out, but remember, once you get it working it 
will be well worth the effort. 

A screenshot of SDR# tuned to WWV is shown in 
Figure 7. It's interesting to see atmospheric fading in the 

44 NUTS \ VOLTS July 2015 



waterfall plot. Running SDR# is like having 
a radio lab at your disposal More details 
on its operation can be found at 
http://sdrsharp.com. 

Now, let's take a look at some of the 
screen buttons. In the upper left corner is 
the Play button — which obviously starts 
the program. What is not obvious is that 
some settings cannot be changed while it is 
running. So, if you cannot set something, 
make sure the program is stopped. The first 
thing you should do is select the USB driver 
you installed by pressing the down arrow 
located next to the Play button. Choose 
RTL-SDR/USB from the available options. 

Next, click Configure. Choose a sample 
rate of 1.024 MSPS and Quadrature 
sampling. Leave the other boxes blank. 
Notice that there is a slider to select the RF 
gain. Set it to about 14.4 dB. It can be 
changed later if it's too low. There is 
another box at the bottom that allows 
correction for the crystal frequency inside 
the RTL, We will discuss that later. 

Now, look below the Play button. 

There are numerous buttons for selecting NFM, AM, and 
so forth. In this screen area is a box labeled Shift. Click 
this box and enter: 24,000,000. Notice the 'minus' sign. 
This number corresponds to the crystal frequency in the 
up-converter and will provide the first rough correction — 
it enables SDR# to read out the RF 5W frequency directly. 

Now, move down to the Audio section. The 
Samplerate and Input boxes should be grayed out. This is 
because data is being sent via USB not audio. In the 
Output box, select the sound card that you are using. It 
will produce the sound you will hear. 

Let all of the other settings in SDR# be at their default 
for now. You will have lots of fun experimenting with them 
later on. Connect a long wire antenna and ground 
counterpoise. Make ihem as long as possible -25 feet or 
more. With any luck, you should now have a functioning 
SW radio. 

Three automatic gain controls (ACC) are available. 

The ones in the Configure box do not work well. The AGC 
on the main screen works well on strong stations. For 
weak stations, leaving AGC off and manually adjusting the 
RF gain may work best. 

Other SDR# settings are usually a matter of 
preference, but here are some guidelines. Generally, a 
moderate value of Zoom should be used unless you are 
calibrating the radio. This makes it easier to click on a 
peak to select a station. Use a small number for FFT — 
typically 4096 to get good computer performance. Use a 
bandwidth that is appropriate for the signal being 
monitored: CW 800 Hz; SSB 2.8 kHz; and AM 10 kHz. A 





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FIGURE 7. SDR# opening screen — radio tuned to 10 MHz WWV. 



filter order of 40 works well and will 
have little effect on program speed. 

Tuning in an 5SB station can be a 
challenge as there is no carrier peak. 

Tune to the left or right of the signal 
depending if USB or LSB is used, and 
adjust the frequency dial slowly until 
voice is intelligible. 

You may want to calibrate the 
frequency dial more accurately. This is 
a two-step procedure as there are two 
crystal oscillators used here: one in the 
RTL and one in the RF converter. To 
calibrate the RTL, uncheck Shift and 
connect a short wire of three or four 
feet directly to an RTL antenna 
terminal. This should allow you to pick 
up a NOAA weather station in your 
area — usually around 162.400 MHz. 

Look up exact frequencies on the 
Internet. Note that NOAA uses NFM, 

Click Configure and adjust the 
Frequency Correction (PPM) until the 
dial reading corresponds to the NOAA 
frequency. 

To calibrate your HF SDR receiver, 
check Shift and tune to station WWV using USB — not 
AM. WWV at 10 MHz is useful for calibrating. Use a high 
value of FFT resolution — around 131,073 — so you can 
see WWV peak dearly in the spectrum. Adjust the Shift 
value until the dial reads the peak frequency accurately to 
within 50 Hz. tt will drift periodically, but that is the nature 
of uncompensated crystal oscillators. 

As noted above, if you bypass the HF converter and 
use an antenna directly connected to the RTL, you can 
take advantage of the very large frequency range of this 
device — up to 1,766 MHz. Thus, you may be able to 
directly pick up FM stations (and other stations like 
NOAA) and two-meter ham repeaters if they are close 
enough. 

Final Comments 

Hopefully, by now you should be having fun with your 
SDR SW receiver. If you are having difficulties, go to 
http://sdrsharp.corn or the Yahoo SDR group and they 
may be able to help. 

Shortwave radio is more exciting now than ever 
before. Many new AM broadcasters from China, Cuba, 
Europe, and other places will keep you informed and 
entertained. Religious broadcasters also abound. 
Unfortunately, many of the interesting stations only appear 
at night due to propagation conditions. Fortunately, the 
powerful ones come booming in even in the daytime. 

Listening to ham radio operators using SSB and Morse 



code is also fun — especially during contests. Other 
stations are using SSB too, such as Maritime WX on 8.763 
MHz and Aviation WX on 10.051 MHz. These generally 
use synthetic voices. If you are lucky — as I was — you 
may hear one of the mysterious 'numbers' stations using 
USB around 13.199 MHz, However, they are seldom in 
the same spot twice. Also interesting are utility stations 
such as WLO, which transmits on many frequencies and 
provides high seas communication and weather 
information. 

One of the exciting things you can do with this radio 
is receive data transmissions such as WX FAX, S1TOR, 
RTTY, BPSK, WSPR, and Easy Pal S5TV. These modes can 
easily be decoded with available software, but to discuss 
this further would require another magazine article! 

If you like playing around with this radio and find the 
subject fascinating, you may want to consider becoming a 
radio amateur. Hams are involved with building and 
studying receivers, transmitters, antennas, satellites, EME, 
microwaves, and experimenting with new radio modes 
such as WSPR, BPSK, Packet Radio, and more. If you are 
considering joining the fraternity of radio amateurs, the 
ARRL website may be the place to start. Be sure to check 
out the new column here in Nuts & Volts , as well. 

In the meantime, have fun with your new SW radio. 

NV 



July 2015 NUTS S VO ITS 45